Expansion joint for coaxial lines



June 30, 1953 E. J. BERNET 2,644,028

EXPANSION JOINT FOR COAXIAL'LINES Filed May 4, 1942 gvwam bow Edwin JBernet a, y l m and mast.

j resonant transmission lines.

Patented June 30, 1953 EXPANSION JOINT FOR COAXIAL LINES Edwin J.Bernet, Washington, D. 0.

Application May 4, 1942, Serial No. 441,731

4 Claims. (-01. 174-88) (Granted under Title 35, U. S. Code (1952), sec.266) This invention relates broadly to expansion joints and morespecifically to such joints for use in coaxial transmission lines, suchas those used in connection with transceiver antennas.

It is well known that dissimilar substances,

.such as wood and metals, when subjected to variations in temperaturewill expand or contract by different amounts.

There has long existed a need of a device for coaxial transmissionlines, secured to a ships mast or the like, that will compensate fordifferentia1 changes in length of said transmission line Manydifficulties have been encountered in attempting to solve this problem.The

' principal difficulty lies in keeping the characteristic impedance ofthe line constant, or nearly so.

Many devices have been developed to compensate for changes in length ofa coaxial transmission line due to changes in temperature, but they havealso altered the characteristic impedance of the line, thereby settingup standing waves on the line, and resulting in a considerable amount ofreflection. This is an undesirable effect in non- Though my inventiondoes not'completely eliminate this effect it I greatly improves itscondition.

An object of this invention is to compensate for the changes in lengthof a coaxial transmission line, due to wind or other extraneous forceswhich would produce a relative mechanical movement between the lineitself and its support.

A more specific object of this invention is to compensate for thechanges in length of a coaxial my invention;

Fig. 2 is a side View of one section of the transmission line orsplicing section;

Fig. 3 is an end View of the section of transmission line or splicingsection, shown in Fig. 2;

Fig. 4 is an elevational View in cross-section of the expansion joint ofFig. 1, in a contracted state, and

Fig. 5 illustrates, in cross-section, an alternate arrangement of partsconstituting a second embodiment of the invention.

It is Well known to those skilled in the art that a change incharacteristic impedance of a nonresonant coaxial line will producereflection. a coaxial line the characteristic impedance is determined bythe ratio between the inside diameter D1, of the outer conductor to theoutside diameter D2, of the inner conductor.

when air is the dielectric. It is therefore obvious that this ratioshould be kept as nearly constant as possible at all points along theline for best results in energy transmission. The invention succeeds inaccomplishing this result to a greater extent than has before beenrealized.

' In accordance with the invention the coaxial transmission line isprovided, at any convenient point along its length with a splicingsection as shown at B in Fig. 1, comprising a pair of coaxial sleeves 3and 3. The splicing section is shown as inserted between two sections Aand A of the transmission line, the conductors of section A being markedI and 5 and those of section A being marked 2 and T. In Fig. l thesleeves of the splicing section are shown telescoped outside therespective conductors of the line sections over which they have a snugfit. One end of each of the sleeves is welded to its respective lineconductor as shown at 5, while the ends of the conductors of the othersection of the line are slotted as shown at 9 in Fig. 2 to provide aplurality of fingers l0 which spring outwardly to provide a constantelectrical contact between line section A and splicing section Bregardless of the relative movement of the sleeve and conductor. In Fig.1 splicing sleeve section B has a zero setting,

. namely, the ratio of diameters throughout the line and splicingsection remain unchanged.

Inner conductors l, 2 are extended a selected distance beyond the endsof outer conductors 6, l

of main line A-A". Inner conductor 2 is extended beyond the end of outerconductor 7 a dispurposes.

The length of outer sleeve 4 of splicing section B is equal to thedistance between the adjacent ends of outer conductors 5 and 1 plus adistance required for a suitable weld on 1 and the distance needed forcoupling and change in length purposes. The length of inner sleeve 3 ofsplicing section B is equal to the distance between the adjacent ends ofthe outer conductors ii and 1.

The thickness of inner sleeve 3 is selected equal to an amount requiredto keep the ratio of diameters constant for a zero setting of thesplicing section. The inside diameter of outer sleeve l is equal to D1plus twice the thickness of outer conductors t or 'i and the outsidediameter of inner sleeve 3 is equal to D2 l twice the thickness of innersleeve 3. Therefore the thickness of sleeve 3 is selected to maintainthe ratio constant.

From the foregoing description it is obvious that a zero setting mayonly be realized when the respective ends of inner sleeve and outerconductors ii, i lie in the same plane perpendicular to the longitudinalaxis of the line section A A.

For best overall performance of the syst m, splicing section B shouldhave a zero setting for the average temperature to which the line willbe exposed.

The splicing sleeves 3, l ma r be positioned on the inside surfaces oftheir respective conductors i, ii and 2, l as shown by the embodimentillustrated in Fig. 5. Outer conductors t, l protrude beyond the ends ofinner conductors i, 2. Fhe length of outer sleeve i is chosen equal tothe distance between adjacent ends of inner conductor l, 2 at thesplicing section B. The length of inner sleeve is chosen in a waysimilar to that described for choosing the outer sleeve of the previoussystem. This system will give results which equal that of the systempreviously ole-- scribed but due to the difficulties in machiningsleeves 4'. to the desired thickness, the previous system is preferred.

The graduated scale 3 simplifies the positioning of said splicingsection to a zero setting.

If, as shown in Fig. 1, the sleeves i are positioned on the outsidesurfaces of conductors i, 2 and 6, l, fingers ill are formed in outerconductor 6 and inner conductor l. The graduated scale is located on theoutside surface of conductor ii as shown by Fig. 2.

If, as shown in Fig. 5, sleeves 3, ii are positioned on the insidesurfaces of conductors I, 2 and 6, l, fingers l are cut in outer sleevei and inner sleeve 3. The graduated scale 8 is now located on theoutside surface of sleeve l.

When a drop in temperature occurs the line and splicing section willappear something like that shown by the cross-sectional view of Fig. i.

The ends of outer conductor 6 and inner sleeve 3 no longer lie in thesame plane perpendicular to the longitudinal axis of a line section AA.A small physical discontinuity will now exist in the splicing section B.The ratio of diameters has changed to D1 plus twice the thickness ofconductor 6 to D2. A small percentage of reflection Will ensue. However,due to the inherent rodeo- 1 tion, principally caused by insulators, thereflection due to physical discontinuity will be almost negligible.

In comparative tests my method of length compensation has produced from2- l% reflection for a line length of 100 ft. and a temperature range of0-120 C., while other methods have produced from 2060% reflection underthe same condi tions.

Normally in practice there are many incidental features employed in theconstruction of a splicing section of this type, but they do not formpart of my invention. A tubular bellows is commonly placed over part ofthe coaxial surface of line A and splicing section B to protect saidline surface from oxidizing and insures good electrical contact.Normally a gas seal is provided around the edge of outer conductor d toprevent gas from escaping out of the coaxial line and inflating thebellows. Often a ring stop is provided on the surface of conductor 6 toprevent splicing member B from being wedged down too deep onto coaxialline section A.

Although I have shown and described certain and specific embodiments ofmy invention, I am fully aware that many modifications thereof arepossible. My invention is not to be restricted except insofar as isnecessitated by the prior art and by the spirit of the appended claims.

The invention described herein may bemanufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

I claim:

1. Means for compensating for changes in the length of a coaxialnon-resonant transmission line, due to changes of temperature,comprising a pair of similar, coaxially aligned transmission linesections and means for joining the adjacent ends of said sections, theadjacent ends of the inner conductors of said sections protruding beyondthe ends of the outer conductors of their respective sections, saidjoining means comprising an inner cylindrical sleeve snugly fitted oversaid adjacent ends of said inner conductor, and an outer cylindricalsleeve snugly fitted over adjacent ends of said outer conductor, one endof each of said sleeves being immovably and conductively secured to itsrespective conductor, the other end of each of said sleeves being insliding electrical contact with its respective conductor, the length andpositioning of said inner sleeve being such that at a predeterminedreference temperature in the normal service temperature range of saidline, each end of said inner sleeve will lie in the same transverseplane as each one of said adjacent ends of said outer conductor, theratio between the inside diameter of said outer sleeve and the outsidediameter of said inner sleeve being the same as the ratio between thecorresponding diameters of said conductors.

2. Means for compensating for changes in length of a coaxialnon-resonant transmission line, due to changes in temperature,comprising a pair of similar, coaxially aligned transmission linesections and means for joining adjacent ends of said sections, theadjacent ends of the outer conductors of said sections protruding beyondthe ends of the inner conductors of their respective sections, saidjoining means comprising an inner cylindrical sleeve snugly fitting insaid adj acent ends of said inner conductor, and an outer cylindricalsleeve snugly fitting in said adjacent ends of said outer conductor, oneend of each of said sleeves being immovably and conduotively secured toits respective conductor, the other end of each of said sleeves being insliding electrical contact with its respective conductor, the length andpositioning of said outer sleeve being such that at a predeterminedreference temperature in the normal service range of said line, each endof said outer sleeve lies in the same transverse plane as each of saidadjacent ends of said inner conductor, the ratio between the insidediameter of said outer sleeve and outside diameter of said inner sleevebeing the same as the ratio between the corresponding diameters of saidconductors.

3. Means for compensating for changes in length of a coaxialnon-resonant transmission line, due to changes in temperature,comprising a pair of similar, coaxially aligned transmission linesections, and means for joining adjacent ends of said sections, the endsof one conductor of each of said sections protruding beyond the ends ofthe other conductor of their respective sections, said joining meanscomprising an inner cylindrical sleeve snugly telescoping the adjacentends of the inner conductor of said sections and an outer cylindricalsleeve snugly telescoping the adjacent ends of the outer conductor ofsaid sections, one end of each of said sleeves being immovably andconductively secured to its respective conductor, the other end of eachof said sleeves being in sliding electrical contact with its respectiveconductor, the length and positioning of one of said sleeves being suchthat at a predetermined reference temperature in the normal servicetemperature range of said line, each end of said one of said sleeveswill be in the same transverse plane as each one of the adjacent ends ofone of said conductors, the ratio between the inside diameter of saidouter sleeve and the outside diameter of said inner sleeve being thesame as the ratio between the corresponding diameters of saidconductors.

4. Means for compensating for changes in length of a coaxialnon-resonant transmission line, due to changes in temperature,comprising a pair of similar transmission line sections having similarcharacteristic impedances, and means for joining adjacent ends of saidsections, the adjacent ends of one conductor of said sections protrudingbeyond the adjacent ends of the other conductor of said sections, saidjoining means comprising an inner cylindrical sleeve snugly telescopingthe adjacent ends of the inner conductor of said sections, and an outercylindrical sleeve snugly telescoping the adjacent ends of the outerconductor of said sections, one end of each of said sleeves beingimmovably and conductively secured to its respective conductor, theother end of each of said sleeves being in sliding electrical contactwith its respective conductor, the length and positionin of one of saidsleeves being such that at a predetermined reference temperature in thenormal service temperature range of said line, each end of said one ofsaid sleeves will b in the same transverse plane as each one of theadjacent ends of one of said conductors, and means provided foraccurately positioning said sleeves at the reference temperature, theratio between the inside diameter of said outer sleeve and the outsidediameter of said inner sleeve being the same as the ratio between thecorresponding diameters of said conductors.

EDWIN J. BERNET.

References Cited in the me of this patent UNITED STATES PATENTS NumberName Date 1,037,522 Pindtershoien Sept. 3, 1912 1,841,473 Green Jan. 19,1932 1,921,117 Darbord Aug. 8, 1933 1,929,878 Clavier Oct. 10, 19331,937,652 Green Dec. 5, 1933 2,044,580 Leach June 16, 1936 2,165,961Cork et a1 July 11, 1939 2,249,443 Tringham July 15, 1941 2,404,797Hansen July 30, 1946 2,427,752 Strempel et a1 Sept. 23, 1947 2,437,067Bingley Mar. 2', 1948 FOREIGN PATENTS Number Country Date 17,402 GreatBritain of 1890

